U.S. Patent Publication No. 2012/0034150 A1, published Feb. 9, 2012, the disclosure of which is hereby incorporated herein in its entirety by this reference, discloses background information hereto.
Additional information is disclosed in the following documents, the disclosure of each of which is hereby incorporated herein in its entirety by this reference:                1. International Application No.—PCT/US2013/000072, filed Mar. 15, 2013, now WO 2013/158156 A1, published Oct. 24, 2013, for “Methods and Structures for Reducing Carbon Oxides with Non Ferrous Catalysts,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,702, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        2. International Application No.—PCT/US2013/000076, filed Mar. 15, 2013, now WO 2013/158159 A1, published Oct. 24, 2013, for “Methods and Systems for Thermal Energy Recovery from Production of Solid Carbon Materials by Reducing Carbon Oxides,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,573, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        3. International Application No.—PCT/US2013/000077, filed Mar. 15, 2013, now WO 2013/158160 A1, published Oct. 24, 2013, for “Methods for Producing Solid Carbon by Reducing Carbon Dioxide,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,723, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        4. International Application No.—PCT/US2013/000073, filed Mar. 15, 2013, now WO 2013/15857 A1, published Oct. 24, 2013, for “Methods and Reactors for Producing Solid Carbon Nanotubes, Solid Carbon Clusters, and Forests,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,753, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        5. International Application No.—PCT/US2013/000075, filed Mar. 15, 2013, now WO 2013/158158 A1, published Oct. 24, 2013, for “Methods for Treating an Offgas Containing Carbon Oxides,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,513, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        6. International Application No.—PCT/US2013/000071, filed Mar. 15, 2013, now WO 2013/15855 A1, published Oct. 24, 2013, for “Methods for Using Metal Catalysts in Carbon Oxide Catalytic Converters,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,848, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        7. International Application No.—PCT/US2013/00008, filed Mar. 15, 2013, now WO 2013/158161 A1, published Oct. 24, 2013, for “Methods and Systems for Capturing and Sequestering Carbon and for Reducing the Mass of Carbon Oxides in a Waste Gas Stream,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,462, filed Apr. 16, 2012, in the name of Dallas B. Noyes;        8. International Application No.—PCT/US2013/000079, filed Mar. 15, 2013, now WO 2013/162650 A1, published Oct. 31, 2013, for “Carbon Nanotubes Having a Bimodal Size Distribution,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/637,229, filed Apr. 23, 2012, in the name of Dallas B. Noyes.        
Ammonia is an important chemical having many applications, such as in the production of fertilizers, cleaners, explosives, etc. Ammonia is directly or indirectly used in a variety of chemical processes to produce various nitrogen-containing compounds, such as amines, aramid fibers, and pharmaceuticals. The production of ammonia is therefore a major worldwide industry. Ammonia is commonly produced by the Haber-Bosch process.
In the Haber-Bosch process, ammonia is synthesized by the reaction of hydrogen and nitrogen in the presence of a catalyst, such as iron, according to Reaction 1:3H2(g)+N2(g)2NH3(g)  (1).The rate of reaction of hydrogen and nitrogen in Reaction 1 is a function of the reaction conditions including the temperature, pressure, and presence of catalyst. Increasing the temperature increases the reaction rate, but also shifts the reaction equilibrium. The equilibrium constant Keq, defined as the ratio of the product of the partial pressures of the product to the product of the partial pressures of the reactants, as shown in the equation
            K      eq        =                  p                  NH          3                2                              p                      N            2                          ⁢                  p                      H            2                    3                      ,is also a function of temperature. However, because Reaction 1 consumes four moles of gas to produce two moles of ammonia gas, the equilibrium conversion to ammonia gas increases with increased pressure. That is, at a given temperature, the fraction of molecules of ammonia present at equilibrium is higher at relatively high pressure than at relatively low pressure. Conventional production of ammonia by the Haber-Bosch process generally involves temperatures between about 300° C. and about 550° C. and pressures between about 5 MPa and about 35 MPa. Conventional production of ammonia is described in, for example, G. Ertl, “Primary Steps in Catalytic Synthesis of Ammonia,” J. Vac. Sci. Technol. A 1(2), p. 1247-53 (1983).
The conditions conventionally used to form ammonia require high-pressure reaction vessels, pipes, valves, and other equipment. Equipment and machinery capable of operating at high pressures have high capital costs because stronger materials (e.g., thicker walls, exotic materials, etc.) are generally more expensive. Furthermore, heating and pressurizing reactants generally require heat exchangers, pumps, and compressors, such that energy consumption may play a significant role in production costs.
Hydrogen used in Reaction 1 may be from any source, but is conventionally formed from methane, coal, or another hydrocarbon. The preparation of the feed gases is typically a multi-step process including steam reforming, shift conversion, carbon dioxide removal, and methanation, with associated apparatus and operating expenses. For example, a common synthesis route is to form hydrogen from methane. In such a process, the methane is reformed typically in a steam reformer, wherein methane reacts with water in the presence of a nickel catalyst to produce hydrogen and carbon monoxide:CH4+H2O→CO+3H2  (2),which is referred to in the art as a “steam-reforming” reaction. Secondary reforming then takes place using oxygen to convert residual methane to carbon oxides, hydrogen, and water:2CH4+O2→2CO+4H2  (3);CH4+2O2→CO2+2H2O  (4).Carbon monoxide is then converted to carbon dioxide to form additional hydrogen:CO+H2O→CO2+H2  (5),which is referred to in the art as the “water-gas shift reaction.” Carbon dioxide is removed from the mixed gases and is typically discharged to atmosphere. The gases are then passed through a methanator to convert residual carbon monoxide, a catalyst poison, to methane and water:CO+3H2→CH4+H2O  (6).The overall result of Reactions 2 through 6 is that methane and steam are converted to carbon dioxide and hydrogen. Conventional preparation of hydrogen from hydrocarbons, such as described for the example of methane, for use in the Haber-Bosch process may be performed in a series of reactors, and may require separation or other treatment of some components of gas streams to fonu a suitably pure hydrogen stream.
Ammonia production as outlined above results in significant releases of carbon dioxide to the atmosphere. Concerns with regard to anthropogenic greenhouse-gas emissions make such emissions undesirable. Thus, it would be advantageous to provide a method of forming ammonia that minimizes or eliminates carbon dioxide emissions.
Separation of carbon dioxide from exhaust gases, such as from combustion sources, process offgases, etc., is becoming a significant concern in mitigating anthropogenic greenhouse-gas emissions. Such streams typically include carbon dioxide in a mixture of other gases, particularly nitrogen (e.g., in combustion effluents) and often hydrogen (e.g., in synthesis gases). Separation of the carbon dioxide from other gases and the transport of the resulting carbon dioxide typically include liquefaction of the carbon dioxide, which is costly. Eliminating the need for the separation of carbon dioxide from nitrogen, carbon monoxide, methane, and hydrogen would be of significant benefit for many types of exhaust gases. Subsequently processing the gas mixture into valuable products, such as solid carbon and ammonia, could alleviate or eliminate some emissions of carbon dioxide. The conversion of carbon dioxide to solid carbon products may have value from the perspective of carbon capture and storage.